Methods and compositions for cleaning metal



United States Patent 3,260,619 METHODS AND CUMPOSITIONS FOR CLEG METALRobert H. Shoemaker and Harold Geschwender, Detroit, Mich, assignors toKolene Corporation, Detroit, Mich. No Drawing. Filed Feb. 4, 1965, Ser.No. 430,465 14 (Claims. (Cl. 134-4)) This is a continuation-in-part ofour earlier application of the same title, Serial No. 241,169, filedNovember 30, 1962, now abandoned.

This application relates to methods and compositions for cleaning metaland relates more particularly to the cleaning of a large variety ofnormally difficult metal surfaces.

As conducive to a better understanding of the instant inventive concept,it should be recognized that various materials in the form of fusedmolten alkali salts have been used heretofore for removing oxide scaleand the like from metal surfaces, such as the well known mixturedisclosed in Webster et al. US. Patent No. 2,458,661 known in the tradeas Kolene Kl. While such materials are extremely efiicient for manyapplications, it has been found that, with certain particularlydifficult problems, prior art compositions and methods are somewhatlacking. For example, stainless steel tubing and the like is frequentlymanufactured by an extrusion or drawing process utilizing glass ofvarious forms as a lubricant. Such procedures are described in detail inSejournet et al. Patent No. 2,538,917 among others. The resultingprodnot not only has an oxide scale formed on its surface from the hightemperatures utilized, both on its interior and exterior in the case oftubing, but further includes a tenaciously bonded, fused, vitreouscoating which is frequently uneven, ranging in thickness from a fewthousandths of an inch to as much as A inch in places. Many mechanicalmeans such as sandblasting and the like are used industrially for theremoval of this coating, but it will be readily seen that, in additionto the inherent difficulties of such a procedure, the stainless steelsurface resulting therefrom may well be pitted or otherwise marred.Although certain molten alkali baths such as K-l are useful in thecleaning of surfaces subjected to this type of treatment, it may benecessary to immerse the metallic product from six to eight times beforeall of the glass and oxide is removed. This thus-formed glass coatinghas been found more difficult to clean than siliceous materialsresulting from casting molds or other investment procedures.

In addition to the cleaning of glass-coated metals resulting fromSejournet and other like processes, various other difficult problemshave been encountered utilizing prior art materials. For example, theremoval of scale from titanium, while possible with compositionscurrently in use, presents unique problems in that at the elevatedtemperatures of 900 F. and above necessary for efficient cleaningheretofore, the metallic surface has a tendency to pick up hydrogenresulting in a phenomenon known as hydrogen embrittlement whichdetrimentally affects the properties of the metal thereby necessitatingthe use of a vacuum annealing. Prior art materials have been tried attemperatures below 900 F., but at these relatively low treatingtemperatures, in addition to a sub stantial decrease in the cleaningeffectiveness of the salts, the molten bath has become extremely viscousrendering the same particularly difficult to desludge. In order toprovide a commercially feasible metal cleaning procedure, it isdesirable to utilize a molten salt bath of a relatively low viscositywhich is capable of having the sludge, in the form of oxides and thelike removed from the metal surfaces, continuously displaced byagitation or the like 3,260,619 Patented July 12, 1966 "ice in a mannersuch as disclosed, for example, in Shoemaker et al. US. Patent No.2,863,465 whereby the effectiveness of the bath will be realized for anextended period of time whether the cleaning operation is of the batch,or continuous, type. Prior art materials have been particularlyineflicient, both from a cleaning and from a desludging standpoint,unless the relatively high temperatures mentioned hereinabove wereutilized. These problems are somewhat magnified with the use of variousalloys of titanium which are more difiicult to descale than puretitanium and with which it has been necessary to use temperaturessubstantially in excess of 900 F., and sometimes even above 1000 F. inorder to provide complete descaling.

Similarly, many of the highly sophisticated alloys known generally inthe trade as superalloys or wrought heatresisting alloys areparticularly difiicult to clean utilizing prior art materials. Suchalloys have high oxidative resistance and tensile strength attemperatures up to about 2500 F. and contain additives such as chromiumto combat oxidation, nickel, manganese, nitrogen, molybdenum or tungstento render the same austenitic and cobalt, titanium, aluminum and siliconto further provide superstrength and other desirable characteristics foruse in high pressure industrial equipment, jet-engines, missiles and thelike. Once again, the use of normal temperature ranges in excess of 900F. necessary with prior art materials produces a product having manydeficiencies. For example, superalloys descaled in this manner aresubject to pitting, possible galvanic attack in the salt, particularlywith molybdenum-bearing alloys, and possible age hardening. Further,prior art compositions at these high temperatures have a tendency toform more oxides on the metallic surfaces than they remove as contrastedwith the reverse in the instant case. Attempts to use lower temperatureswith these superalloys result in inefficient descaling and undueviscosity of the molten bath rendering the same difiicult to desludge.

It is a primary object of the instant invention to provide compositionsand methods for the cleaning of various metals which are free from theforegoing, and other, disadvantages.

Another object of this invention is the provision of fused,substantially anhydrous, molten alkali salt mixture which isparticularly adapted for exceptionally difficult cleaning processes.

A further objective of the instant invention is to provide a material ofthe type described having a composition uniquely capable of use in alarge variety of different procedures without modification.

A particularly. important object of this invention is the provision of acomposition having the ability to efficiently clean particularlysensitive metallic surfaces at relatively low temperatures, for example,below 900 F., and preferably below 800 F., without becoming undesirablyviscous and difiicult to desludge.

Yet another object of the instant invention is to provide a method forboth descaling and deglassing materials manufactured utilizing avitreous lubricant at high temperatures.

A further object of this invention is the provision of methods fordescaling titanium and alloys containing a predominant portion oftitanium which preclude hydrogen pickup thereby avoiding the necessityof relatively expensive protective treatments such as vacuum annealing.

A still further object of this invention is the provision of methods forcleaning the surfaces of wrought heatresisting steel alloys in such amanner as to preclude deleterious effect on the characteristics of theProduct.

Additionally, it is an important object of this invention to provide acomposition and methods for the cleaning of metals which are highlyefiicient and reliable in operation,

relatively inexpensive and simple in practice, and particularlynoncorrosive to the equipment being utilized.

Other and further objects reside in the particular com bination andquantitative ratios of the component parts of the compositions and thespecific arrangement of the manipulative steps of the methods.

Consistent with the above objectives, it has been found that a fused,substantially anhydrous cleaning material formed of the followingmixture has all of the desired properties and characteristics:

Of particular importance in the formation of the compositions to beutilized according to the methods of the instant invention is the weightratio of potassium hydroxide to sodium hydroxide. While many prior artpatents generally suggest the use of one or both materials, it has notbeen recognized heretofore that the potassium hydroxide provides uniqueadvantages to the composition. Potassium hydroxide is more aggressiveand dissolves the iron in iron oxide scales more readily than sodiumhydroxide baths. Similar compositions without the potassium hydroxidehave been found to be particularly inefficient and undesirable in thatthey fail to provide the excellent descaling and deglassing propertiesof the material of the instant invention and further in that they arefound to sometimes even oxidize the metallic surface being treatedrather than remove the oxides. A weight ratio between these constituentsof at least 0.1 has been ascertained to be necessary to render thecomposition effective as a multipurpose cleaning material andparticularly with regard to its efl iciency in the descaling of titaniumand certain ditficult titanium alloys. Further, the presence of thepotassium hydroxide in at least this quantitative ratio has provided theunique descaling-deglassing properties for cleaning metal surfacesmanufactured utilizing a vitreous lubricant. Also, the maintenance ofthis potassium hydroxide level renders the material especially effectivefor cleaning other normally difiicu-lt metallic surfaces such as thoseof wrought heat-resisting steel superalloys. The upper level of theweight ratio between potassium hydroxide and sodium hydroxide in thecomposition may be somewhat varied, but it has been found that a valueof substantially in excess of 0.4 is undesirable from a commercialviewpoint. The advantageous cleaning properties provided by the presenceof the potassium hydroxide must be balanced against its increased costas compared with sodium hydroxide and its corrosive effect on both theequipment being utilized and the material being treated.

The presence of the sodium nitrate in the molten alkali bath has alsobeen found to be critical to the production of an efiicient descalingand/ or deglassing technique. This material in the specific mixturedefined hereinabove contributes to the unique properties of the bath andits capability of use for many varied cleaning purposes.

It will be noted that the sodium'chloride is included in a relativelyhigh percentage as compared to the one or two percent sodium chlorideremaining in commercially produced caustic from its manufacturingprocess. This material assists in providing a molten bath which remainsrelatively thin or of low viscosity even at somewhat reduced treatingtemperatures such as below 900 F. or even below 800 F. As pointed outhereinabove, this property of the molten bath of this invention isextremely important in order to provide a commercially acceptable methodof continuously desludging the same thereby extending its effective lifeand precluding undesirable contamination of the product.

Although each of the materials forming part of the bath of thisinvention are known in and of themselves for use in the cleaning ofmetallic surfaces, the particular combination defined hereby and thespecific quantitative relationship between the components of themixture, provide a synergi tic result not realizable from the individualmaterials or other combinations of the same. Specifical- 1y, thisparticular composition has the advantage of being useful withoutmodification for cleaning in an efficient manner various metallicsurfaces which have caused difiiculty in the past.

While it is particularly important to include all of the materials inthe quantitative ratios set forth hereinabove, it is also of greatimportance to exclude various materials found in previously knowncompositions. For example, certain baths suggested by the prior art haverequired the presence of additional components such as alkalicarbonates. However, it has been found that baths of this compositionmust be frequently replaced because their eifectiveness has deterioratedbelow a point of minimum usefulness. This relatively rapid decrease inefficiency of the bath is a direct result of the presence of thecarbonate. Similarly, the inclusion of various other materials has beenfound to produce deleterious effects on the bath in many instances.Hydrogen-containing compounds, such as hydrides and the like, render thebath undesirable for titanium cleaning in that the effects of hydrogenembrittlement are increased. Thus, the specific composition defined bythis invention, and this composition alone, can provide the uniquemultipurpose characteristics desired.

With regard to the treating methods, the metal to be cleaned is immersedin a molten bath of the above composition for a period of time which maybe as little as 30 seconds while the bath is maintained at an elevatedtemperature. It will be understood that the time of the treatment willdepend upon the size of the work-piece as well as the type and thicknessof the material to be removed and the temperature of the bath. While thetemperature may vary over a substantial range of from above its meltingpoint to in excess of 1000 F., the specific composition of the bath ofthis invention allows the same to be utilized at relatively lowtemperatures, below 900 F. and even below 800 F. in those instanceswhere higher temperatures would be undesirable such as in the treatmentof titanium, titanium alloys, and steel superalloys. This ability avoidsaging of the material and various other disadvantageous phenomenona suchas hydrogen pickup, pitting, oxidation and the like as explainedhereinabove.

Following the immersion, the metal is withdrawn from the alkali bath,preferably washed in water to remove any of the bath clinging to itssurfaces, and pickled in a conventional manner in a dilute acidicmaterial such as sulfuric acid, hydrochloric acid which may be in theform of sodium chloride added to the sulfuric acid, nitric acid,hydrofluoric acid and the like, alone or in admixtures, maintained atelevated temperatures, for example, in excess of about F. to provide aclean bright surface. The pickling treatments may vary substantially inaccordance with well known techniques.

The bath of this invention maintains its efiiciency over extendedperiods, small quantities of additional material being added from timeto time to replace losses occurring from dragout of the workpieces andtheir carriers, these intermittent additions maintaining both the volumeand equilibrium of the bath.

Having now described the instant inventive concept in more generalterms, the following specific examples will serve to illustrate the samealthough, it is to be understood that these examples are not to beinterpreted as limiting.

Example 1 A 2600 pound charge of a material having the preferredformulation set forth above was added to a salt pot and heated to atemperature of approximately 900 F. Sections of glass-coated, scaledtubing inches long by 3 /2 inches diameter having nonuniform vitreousbonded and fused coatings on their surfaces as much as inch thick inplaces resulting from a Sejournet-type extrusion process, were immersedin the bath for periods of times ranging from 30 seconds to severalhours. The treated tubes were then withdrawn from the bath, quenched inwater and pickled in a dilute acidic material in a conventional manner.Substantially all of the scale and glass were removed inmost instancesin a single immersion. In extremely difiicult cases, a singlereimmersion functioned to completely clean the surfaces, the finalproduct having a bright, unmarred finish. Sludge was readily removedduring the processing.

It was found that similar workpieces treated with prior artcom-positions required six to eight immersions before the surfaces wererendered substantially clean.

Example 2 The procedure of Example 1 was repeated immersing titaniumworkpieces having scaled surfaces formed by high temperature annealingprocesses in excess of 1200 C. and as high as 1450 C. The bathtemperature was maintained between 800 F. and 900 F. The resultingproducts were completely descaled with no hydrogen pickup. Even reducingthe temperature to from 700 to 800 F. resulted in excellent descalingwhile the bath viscosity remained sufiiciently low to permitsatisfactory desludging.

Similar titanium samples processed utilizing prior art materialsrequired temperatures in excess of 900 F. to produce satisfactorydescaling with the material picking up hydrogen resulting in detrimentaleffects from the standpoint of the physical characteristics of thetitanium metal in subsequent operations.

Example 3 A procedure similar to Example 2 was utilized to descaleparticularly diflicult alloys of titanium such as the following:

Type 48-1 containing 8 percent manganese;

Type 461 containing 5 percent aluminum and 2.5 percent tin;

Type 464 containing 6 .percent aluminum and 4 percent vanadium; and

Type 120 VCA containing 13 percent vanadium, 11 percent chromium and 3percent aluminum.

Completely satisfactory descaling of the above materials was realized attemperatures below 900 F. and even below 800 F.

Subjection of these same materials to prior art descaling techniquesresulted in age hardening and other undesirable effects.

Example 4 Utilizing a procedure similar to Example 2, various samples ofsteel superalloys conditioned by heat treating and hot rolling therebyforming oxide coatings, were descaled, such as the following:

Multimelt (M)chromium 18-22 percent, nickel 18-22 percent, molybdenum2.75-3.75 percent, tungsten 2-3 percent, cobalt 18-22 percent, columbium.75-l.5 percent, the rest essentially iron;

Hastelloy X-chromium 20-23 percent, nickel 46 percent, molybdenum 8-10percent, tungsten .2-1 percent, cobalt .5-2.5 percent, iron 17-20percent;

Hastelloy B-molybdenum 26-30 percent, iron 4-6 percent, the restessentially nickel;

Hastelloy Cchromium 13-17 percent, molybdenum 16-18 percent, tungsten3.7-5.3 percent, iron 4.5-7 percent, the rest essentially nickel;

Haynes Alloy chromium 19-21 percent, nickel 9-11 percent, tungsten 14-16percent, iron 2 percent maximum, the rest essentially cobalt; and

Ren 4lchromium 11.5 percent, nickel 76 percent, silicon 4 percent, otherelements 5 percent.

Complete descaling of these materials at temperatures below 900 F. andeven below 800 F. has been effected with the salt remaining thin andfluid thereby facilitating desludging of the same.

Similar treatments of these and comparable superalloys with prior artcompositions necessitated the use of temperatures in excess of 900 F.resulting in pitting, age hardening, galvanic attack and oxidation bythe salt.

It will now be seen that there are herein provided compositions andmethods for the cleaning of metals, specifically a large variety ofnormally difficult materials at relatively low temperatures, whichsatisfy all of the objectives of the instant application, and others,including many advantages of great practical utility and commercialimportance.

Since many embodiments may be made of the instant inventive concept, andsince many modifications may be made of the embodiments hereinbeforedescribed, it is to be understood that all matter herein is to beinterpreted merely as illustrative and not in a limiting sense.Accordingly what is claimed is:

1. A composition for the cleaning of metals consisting essentially of afused, substantially anhydrous, mixture of from 9-15 weight percentsodium nitrate, from 5-15 weight percent sodium chloride, from 8-20weight percent potassium hydroxide and from 50-78 weight percent sodiumhydroxide, the weight ratio of potassium hydroxide to sodium hydroxidebeing at least 0.1 and not substantially in excess of 0.4.

2. A composition in accordance with claim 1 wherein said mixtureconsists essentially of 12 weight percent sodium nitrate, '10 weightpercent sodium chloride, 15 weight percent potassium hydroxide and 63weight percent sodium hydroxide.

3. A method for cleaning stainless steel and titanium base metalscomprising immersing the metal to be treated in a molten bath consistingessentially of a mixture of from 9-15 weight percent sodium nitrate,from 5-15 weight percent sodium chloride, from 8-20 weight percentpotassium hydroxide and from 50-78 weight percent sodium hydroxide, theweight ratio of potassium hydroxide to sodium hydroxide being at least0.1 and not substantially in excess of 0.4, for at least 30 seconds,maintaining said bath at an elevated temperature above its melting pointduring the immersion, and removing the metal from said bath.

4. A method in accordance with claim 3 wherein said mixture consist-sessentially of 12 weight percent sodium nitrate, 10 weight percentsodium chloride, 15 weight percent potassium hydroxide and 63 weightpercent sodium hydroxide.

5. A method in accordance with claim 3 further including the steps ofwashing the metal removed from said bath in water, and subjecting thesame to a pickling treatment in a dilute acid solution.

6. A method of removing scale and glass from a stainless steel productmanufactured by a high temperature process utilizing glass as alubricating material comprising immersing the stainless steel product ina molten bath consisting essentially of from 9-15 weight percent sodiumnitrate, from v5-15 weight percent sodium chloride, from 8-20 weightpercent potassium hydroxide and from 50-78 weight percent sodiumhydroxide, the weight ratio of potassium hydroxide to sodium hydroxidebeing at least 0.1 and not substantially in excess of 0.4 for at least30 seconds, maintaining said bath at a temperature above its meltingpoint during the immersion, and removing the stainless steel productfrom the bath.

7. A method in accordance with claim 6 wherein said mixture consistsessentially of 12 weight percent sodium nitrate, 10 weight percentsodium chloride, 15 weight percent potassium hydroxide and 63 weightpercent sodium hydroxide.

8. A method in accordance with claim 6 further including the steps ofwashing the stainless steel product in water, and subjecting the same toa pickling treatment in a dilute acid solution.

9. A method of descaling a metal selected from the group consisting oftitanium and alloys containing a predominant portion of titaniumcomprising immersing the metal to be treated in a molten bath consistingessentially of a mixture of from 9- 15 weight percent sodium nitrate,from 515 weight percent sodium chloride, from 8-20 weight percentpotassium hydroxide and from 50-78 weight percent sodium hydroxide, theweight ratio of potassium hydroxide to sodium hydroxide being at least0.1 and not substantially in excess of 0.4 for at least 30 seconds,maintaining said bath at a temperature above its melting point and belowabout 900 F. during the immersion, and removing the metal from saidbath.

10. A method in accordance with claim 9 wherein said mixture consistsessentially of 12 Weight percent sodium nitrate, 10 weigh-t percentsodium chloride, 15 weight percent potassium hydroxide and 63 weightpercent sodium hydroxide.

11. A method in accordance with claim 9 wherein said temperature ismaintained at from 700800 F.

12. A method of descaling a wrought heat-resisting steel alloycomprising immersing the alloy to be treated in a molten bath consistingessentially of a mixture of from 9-1'5 weight percent sodium nitrate,from 5-15 weight percent sodium chloride, from 8-20 weight percentpotassium hydroxide and from 5078 weight percent sodium hyroxide, theweight ratio of potassium hyroxide to sodium hydroxide being at least0.1 and not substantially in excess of 0.4 for at least 30 seconds,maintaining said bath at a temperature above its melting point and belowabout 900 F. during the immersion, and removing the alloy from saidbath.

13. A method in accordance with claim 12 wherein said mixture consistsessentially of 12 weight percent sodium nitrate, 10 weight percentsodium chloride, 15 weight percent potassium hydroxide and 63 weightpercent sodium hydroxide.

14. A method in accordance with claim 12 wherein said temperature ismaintained at from 700800 F.

References Cited by the Examiner UNITED STATES PATENTS 2,655,481 10/1953Clingan 252-156 XR 2,931,778 4/1960 Ilenda et a1 252156 3,030,239 4/1962Mekjean et al. 252 156 XR FOREIGN PATENTS Y 490,861 2/1953 Canada.

JULIUS GREENWALD, Primary Examiner.

J. T. FEDIGAN, Assistant Examiner.

6. A METHOD OF REMOVING SCALE AND GLASS FROM A STAINLESS STEEL PRODUCTMANUFACTURED BY A HIGH TEMPERATURE PROCESS UTILIZING GLASS AS ALUBRICATING MATERIAL COMPRISING IMMERSING THE STAINLESS STEEL PRODUCT INA MOLTEN BATH CONSISTING ESSENTIALLY OF FROM 9-15 WEIGHT PERCENT SODIUMNITRATE, FROM 5-15 WEIGHT PERCENT SODIUM CHLORIDE, FROM 8-20 WEIGHTPERCENT POTASSIUM HYDROXIDE AND FROM 50-78 WEIGHT PERCENT SODIUMHYDROXIDE, THE WEIGHT RATIO OF POTASSIUM HYDROXIDE TO SODIUM HYDROXIDEBEING AT LEAST 0.1 AND NOT SUBSTANTIALLY IN EXCESS OF 0.4 FOR AT LEAST30 SECONDS, MAINTAINING SAID BATH AT A TEMPERATURE ABOVE ITS MELTINGPOINT DURING THE IMMERSION, AND REMOVING THE STAINLESS STEEL PRODUCTFROM THE BATH.
 8. A METHOD IN ACCORDANCE WTH CLAIM 6 FURTHER INCLUDINGTHE STEPS OF WASHING THE STAINLESS STEEL PRODUCT IN WATER, ANDSUBJECTING THE SAME TO A PICKLING TREATMENT IN A DILUTE ACID SOLUTION.